The invention relates to electron multipliers. In particular, the invention relates to continuous dynode, channel electron multipliers employing a zener diode biasing arrangement.
Particle detectors employing channel electron multipliers (CEM) are well known. As used herein, the term CEM covers known single channel and multiple channel arrangement. A channel electron multiplier (CEM) is an example of a known single channel electron multiplier device. Such a device employs a continuous dynode which is an active surface formed on an interior wall of the channel. A continuous dynode is an active surface capable of supporting electron multiplication and carrying a bias current to replenish emitted electrons. Typically, the continuous dynode comprises a current carrying semiconductive layer which has been activated to support electron multiplication. The semiconductive layer is formed on a substrate such as the interior of a glass tube. A microchannel plate is an example of a multichannel arrangement of continuous dynodes.
An exemplary single channel CEM 10 is shown in FIG. 1. In such an arrangement, an energetic particle 12, such as an ion, an electron, a photon or a neutral particle enters the input end 14 of the device strikes the dynode surface 15, and typically produces two or three secondary electrons 16. These electrons are accelerated down the channel by a bias provided by high voltage source 18. The electrons 16 repeatedly strike the dynode 15 producing additional electrons (and so on) until a pulse of about 10.sup.5 -10.sup.8 electrons 22 emerges at the output end. The pulse of output electrons 22 is detected by a collector 26. For positive ions, the input 14 is generally at a negative potential and the output 24 is at ground. Bias resistor 28 establishes a voltage relationship between the output 24 and the collector 26, which is at ground reference potential. In the exemplary electron multiplier 10, shown in FIG. 1, the dynode is an activated semiconducting layer formed on the interior of glass channel wall 20.
The gain of a channel electron multiplier is defined as a ratio of the output current to the input current. Gain in general is a function of the secondary emission coefficient of the dynode, the applied voltage, the ratio of the length of the channel to its diameter (1/d), and the ratio of the length to its radius of curvature (1/r) if the channel is curved.
FIG. 2 shows a typical gain versus voltage curve for a CEM 10. As the voltage applied to the CEM increases, the gain of the CEM increases. The desired operating point on the curve is determined primarily by whether the CEM is to be operated in the analog or pulse counting mode.
In the analog or current measurement mode, the linearity of the channel electron multiplier is dependent upon the conductivity or bias current capability of the device. Analog channel electron multipliers require an electrically isolated collector and an bias resistor, discussed below, which promotes linear output current generally between about 10 and 30 percent of the bias current.
For proper operation, the channel electron multiplier must be maintained at a positive bias. This means that the collector must be the most positive element in the detector circuit. As illustrated in FIG. 3, which is a schematic diagram of the arrangement of FIG. 1 with similar reference numerals, this may be accomplished by means of the bias resistor 28. The bias resistor 28 establishes a potential difference between the output 24 of the channel electron multiplier 10 and the collector 26, as illustrated. The bias resistor 28 may be a separate external device or it may be integrally formed on the external wall portion of the CEM 10. A disadvantage of integrated resistors is that they are difficult to control precisely. Also, the optimum resistance depends on the overall resistance of the CEM which can vary between devices and also changes with age. As the CEM ages, the gain decreases. Consequently, a higher potential is required across the device to boost the gain to the desired range. For example, in FIG. 3A, an electrical equivalent circuit of the device of FIG. 3. An electrical resistor R.sub.d represents the resistance of the CEM and an electrical resistor R.sub.b represents the bias resistance. By means of simple voltage division: EQU V.sub.o =V.sub.i R.sub.b /R.sub.b +R.sub.d
Unfortunately, as the input voltage V.sub.i is increased in order to maintain the gain it is always divided. Therefore, it is necessary to increase the input voltage V.sub.i by a larger amount than if no voltage division were to occur. These larger increases in input voltage cause the device to reach its maximum operating voltage which is undesirable.
It has been suggested that it would be advantageous if a constant output potential could be maintained. It is possible, for example, to provide a constant voltage on the output of an electron multiplier by means of a separate power supply. However, the CEM is normally used in a vacuum environment. Thus, an additional feedthrough would be required plus the additional cost of the second power supply renders this expedient undesirable. It has also been suggested to employ a forward biased zener diode as a replacement for the bias resistor. However, such a forward biased zener diode would not operate properly to achieve the desired results.